The present invention relates to power transmission mechanism that transmits driving rotation of a driving shaft to a driven member by any one of two friction clutches thereby to rotate the driven member.
Conventionally, power transmission mechanism as disclosed in, for example, Unexamined Published Japanese Patent Application No. Hei 11-82648 has been known, which transmits driving rotation of a driving shaft to a driven member by either a negative type first friction clutch or a positive type second friction clutch thereby to rotate the driven member.
This power transmission mechanism comprises a plural numbers of first and second friction plates constituting a first friction clutch and a second friction clutch respectively; a common piston that can approach and separate from these first and second friction plates, makes the first friction clutch in a connection state when the piston is pressed against the first friction plate and makes the second friction clutch in connection state when the piston separates from the first friction plate and is pressed against the second friction plate; a spring for energizing the common piston so as to press it against the first friction plate; and a fluid passage capable of giving fluid force by which the piston separates from the first friction plate and approaches the second friction plate to the common piston.
In this power transmission mechanism, when the fluid is not supplied to the fluid passage, since the common piston is pressed against the first friction plate by the energizing force of the spring, the negative type first friction clutch is in connection state. At this time, since the common piston separates from the second friction plate, the positive type second clutch is in cut-off state. Next, when the fluid is supplied to the fluid passage, the common piston separates from the first friction plate in opposition to the spring by the fluid force of the fluid, approaches the second friction plates, and is pressed against the second friction plate. As a result, the first friction clutch is switched from the connection state to the cut-off state, while the second friction clutch is switched from the cut-off state to the connection state.
However, in such the conventional power transmission mechanism, when the first and second friction clutches are switched, for example, when the first friction clutch is switched from the connection state to the cut-off state, and when the second friction clutch is switched from the cut-off state to the connection state, as described before, as the pressure of the supplied fluid increases, the pressing force of the common piston against the first friction plate decreases, so that the piston finally separates from the first friction plate. Thereafter, the common piston moves toward the second friction plate freely for a short time and contacts the second friction plates. Then, since the common piston is pressed against the second friction plate by a large pressing force, there exists a period for which the common piston separates from both of the first and second friction plates.
For the period in which the common piston separates from both of the first and second friction plates, and immediately before and after this period, both the first and second friction clutches are in the cut-off state or are substantially in the cut-off state. Therefore, the driven member is substantially cut off from the driving shaft and rotates freely. Here, if the driven member rotates freely as described before, for example, in case that the above power transmission mechanism is applied to a travel driving apparatus of a civil engineering construction machine, when the civil engineering construction machine stops in the middle of a sloping road, there is a problem that the civil engineering construction machine slips on the sloping road by its own weight for a short time.
An object of this invention is to provide power transmission mechanism which can prevent free rotation of the driven member at the switching time of first and second friction clutches.
This object can be achieved by power transmission mechanism that transmits driving rotation of a driving shaft to a driven member by either a negative type first friction clutch or a positive type second friction clutch, thereby to rotates the driven member. In this power transmission mechanism, the first friction clutch comprises a plural numbers of first friction plates; a first piston which can approach and separate from the first friction plate, and makes the first friction clutch in connection state when it is pressed against the first friction plate; and a first spring for energizing the first piston so as to press the first piston against the first friction plate. The second friction plate comprises a plural numbers of second friction plates; a second piston which can approach and separate from the second friction plate, and makes the second friction clutch in connection state when it is pressed against the second friction plate; and a second spring for energizing the second piston so as to separate the second piston against the second friction plate, of which spring constant is smaller than that of said first spring. Further, there is provided a fluid passage which can give the first piston to fluid force by which the first piston is separated from the first friction plate and simultaneously can give the second piston to fluid force by which the second piston approaches the second friction plate.
Now, assuming that the fluid is not supplied to the fluid passage and the fluid force is not given to any of the first and second pistons, since the first piston is pressed against the first friction plate by the energizing force of the first spring, the first friction clutch is in the connection state. Therefore, the rotation of the driving shaft is transmitted through the first friction clutch to the driven member, thereby to rotate the driven member. At this time, since the second piston is separated from the second friction plate by the energizing force of the second spring, the second friction clutch is in the cut-off state.
Next, when the fluid is supplied to the fluid passage, the fluid force in the direction opposite to the energizing force of the first spring is given to the first piston. However, this fluid force increase as the pressure of the fluid increases. Therefore, the pressing force given from the first spring to the first friction plate is canceled by this fluid force and reduced, so that the rotational torque transmitted through the first friction clutch becomes small.
At this time, the fluid force in the direction opposite to the energizing force of the second spring is also given to the second piston. However, the spring constant of this second spring is smaller than that of the first spring as described before. Therefore, before the canceled energizing force of the first spring becomes zero, the above fluid force exceeds the energizing force of the second spring, so that the second piston can be pressed against the second friction plate. Hereby, when the first friction clutch transmits the rotational torque by friction resistance, the second friction clutch is switched to the connection state and starts transmitting the rotational torque. Therefore, there exists a period for which the rotational torque is transmitted to the driven member through both of the first and second friction clutches. The rotational torque transmitted to the driven member from the driving shaft is the sum of the rotational torque of the first friction clutch and that of the second friction clutch.
Thereafter, when the fluid force given to the first piston exceeds the energizing force of the first spring, the first piston separates from the first friction plate and the first friction clutch is switched to the cut-off state. As a result, transmission of the rotational torque which has been performed through both of the first and second friction clutches is performed through only the second friction clutch. On the other hand, contrary to the aforementioned, when supply of the fluid to the fluid passage is stopped, in the order opposite to the aforesaid action, the connection state is changed from only the second friction clutch through both the first and second friction clutches to only the first friction clutch.
At the switching time of the first and second friction clutches, and in the middle of switching them, both the first friction clutch and the second friction clutch are in the connection state as described above. Therefore, the driven member is not cut off from the driving shaft and does not rotate freely. As a result, the civil engineering construction machine is prevented from slipping on the sloping road.
Further, according to the second aspect of the invention, the first and second friction plates can be made the same shape. Therefore, a manufacturing cost can be reduced, and the outer diameter can be made the small diameter while the transmission torque of the power transmission mechanism is being secured.
Further, according to the third aspect of the invention, the rotational speed of the driven member can be switched in two stages with a simple structure.
Further, according to the fourth aspect of the invention, the rotational speed of the driven member can be switched in four stages in total with a simple structure.
Furthermore, according to the fifth aspect of the invention, the driving force is dispersed in the axial direction, and the whole structure can be simplified.
The present disclosure relates to the subject matter contained in Japanese patent application No. Hei. 11-139595 (filed on May 20, 1999), which is expressly incorporated herein by reference in its entirety.